USING POWER TRANSISTORS IN AC SWITCH APPLICATIONS

Power MOSFETs have an anti-parallel body diode which will conduct for every negative cycle of AC signal. BY placing two power MOSFETs source to source, the intrinsic body diodes will prevent each other from conducting. If voltage across MOSFET channel is less than Vf of body diode, then majority of current will flow through the MOSFET channel instead of body diode.

Note: MOSFET channel is bidirectional switch.

The silicon oxide layer between the gate and the source regions can be punctured by exceeding its dielectric strength. Care should be exercised not to exceed the gate-to-source maximum voltage rating. Even if the applied gate voltage is kept below the maximum rated gate voltage, the stray inductance of the gate connection, coupled with the gate capacitance, may generate ringing voltages that could lead to the destruction of the oxide layer. Over voltages can also be coupled through the drain-gate self-capacitance due to transients in the drain circuit.

Body diodes may be utilized as freewheeling diodes for inductive loads. While these diodes usually have rather high forward voltage drop, they can handle large currents and are sufficient in many applications, reducing part count, and thus, device cost and board space.

Extra:Zeners are frequently used “to protect the gate from transients”. Unfortunately they also contribute to oscillations and have been known to cause device failures. A transient can get to the gate from the drive side or from the drain side. In either case, it would be an indication of a more fundamental problem: a high impedance drive circuit. A zener would compound this problem, rather than solving it. Sometimes a zener is added to reduce the ringing generated by the leakage of a gate drive transformer, in combination with the input capacitance of the MOSFET. If this is necessary, it is advisable to insert a small series resistor (5-10 Ohms) between the zener and the gate, to prevent oscillations.

Impedance of Gate Circuit

To turn on a power MOSFET a certain charge has to be supplied to the gate to raise it to the desired voltage, whether in the linear region or in the saturation region. The best way to achieve this is by means of a voltage source, capable of supplying any amount of current in the shortest possible time. If the device is operated as a switch, a large transient current capability of the drive circuit reduces the time spent in the linear region, thereby reducing the switching losses.On the other hand, if the device is operated in the linear mode, a large current from the gate drive circuit minimizes the relevance of the Miller effect, improving the bandwidth of the stage and reducing the harmonic distortion.

MOS-gated transistors should be driven from low impedance voltage sources, not only to reduce switching losses, but to avoid dv/dt induced turn-on and reduce the susceptibility to noise.